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Irrigation Glossary


AC {hertz} abbreviation for alternating current.

AC pipe Asbestos-cement pipe was commonly used for buried pipelines.  It combines strength with light weight and is immune to rust and corrosion.

Acceleration of gravity Acceleration caused by the attraction of the mass of earth to bodies at or near its surface.

Acid Substance with a pH less than 7.0

Adapter A fitting that makes it possible to join different kinds of pipes together in the same run.

Adhesion Physical attraction of unlike substances to one another.  In soils, it is the process that holds water molecules tightly to soil solids at the soil-water interfaces.

Adjusted sodium adsorption ratio Index of permeability problems, based upon water quality.

Adsorption Concentration of a substance at the surface of another, more noticeable with substances of large surface area, such as clay particles.

Advance ratio Ratio of the time for the water to reach the end of the field to the total set time for an irrigation set on a furrow irrigation system.  The ratio should be less than 0.5 to have a good distribution uniformity.

Advance time {minutes, sec}Time required for a given stream of irrigation water to move from the upper end of a field to the lower end.

Aeration To supply or impregnate with air.

Aeration capacity Volume fraction of air filled pores in a soil at field capacity.

Aggregate Groups of individual soil particles, held together naturally and consisting of particles of sand, silt and clay separated from each other by pores, cracks or planes of weakness.  The term, soil structure, refers to this arrangement of the soil in natural aggregates. Various types of soil structure are recognized (Massive, platy, prismatic, blocky, granular).

Air gap Physical separation of the supply pipe by at least two pipe diameters (never less than one inch) vertically above the overflow rim of the receiving vessel. In this case line pressure is lost. Therefore, a booster pump is usually needed downstream, unless the flow of the water by gravity is sufficient for the water use. With an air gap there is no direct connection between the supply main and the equipment. An air gap may be used to protect against a contaminant or a pollutant, and will protect against both back-siphonage and backpressure. An air gap is the only acceptable means of protecting against lethal hazards.

Alfalfa valve Outlet valve attached to the top of a pipeline riser with an opening equal in diameter to the inside diameter of the riser pipe and an adjustable lid or cover to control water flow.

Algicide Substance that will kill or control algae growth.

Alkaline (alkali) soils Soil with pH greater than 7.0.

Allowable depletion That part of soil moisture stored in the plant root zone managed for use by plants, usually expressed as equivalent depth of water in acre inches per acre, or inches.

Allowable stress factor: Coefficient used to modify reference evapotranspiration to reflect the water use of a particular plant or group of plants particularly with reference to the water stress.

Allowable voltage loss Voltage loss in a circuit or portion of a circuit which, if not exceeded, will result in the electrical device working correctly.

Alternate set irrigation  Method of managing irrigation whereby, at every other irrigation, alternate furrows are irrigated, or sprinklers are placed midway between their locations during the previous irrigation.

Alternating current [AC] Current in which the flow of electrons in a circuit flow in one direction and then in the reverse direction.

Ampere Unit of electrical current.  The unit is used to specify the movement of electrical charge per unit time through a conductor.

Anion Negatively charged ion, which during electrolysis is attracted towards the anode.  The most common anions in soil extracts and waters are bicarbonate, sulphate, carbonate, chloride and nitrate ions.

Application rate (also, precipitation rate)

  • Rate at which a sprinkler system applies water to a given area.
  • precipitation rate, lowest Lowest precipitation rate in a defined contiguous area.
  • instantaneous precipitation (application) rate Maximum rate, usually localized, that a sprinkler application device applies water to the soil.
  • net precipitation rate Measure of the amount of water that actually reaches the landscape.  The net precipitation rate is the gross precipitation rate minus the losses that occur between the sprinkler and the landscape surface. 
  • sprinkler precipitation rate {in./h, mm/h} Precipitation rate of a group of heads used together and all having the same arc, spacing and flow.
  • system precipitation rate Precipitation rate for a system is the average precipitation rate of all sprinklers in a given area regardless of the arc, spacing, or flow rate of each head.

Aquifer An aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, silt, or clay) from which groundwater can be usefully extracted using a water well.

Arc Portion of a full circle (360) covered by a part circle sprinkler.

Area Surface included within a set of lines. In irrigation, usually used to describe a surface of land or cross section of pipe.

Arid climate Climate characterized by low rainfall and high evaporation potential.  A region is usually considered as arid when precipitation averages less than 10 inches per year.

Atmospheric pressure Absolute pressure measured at any location.  Standard atmospheric pressure at sea level is defined as 14.7 psi or 34.0 ft of water.

Atmospheric vacuum breaker  Backflow device configured with a single moving part, a float, which moves up or down to allow atmospheric air into the piping system.The AVB is always placed downstream from all shut-off valves. Its air inlet valve closes when the water flows in the normal direction. But, as water ceases to flow the air inlet valve opens, thus interrupting the possible back-siphonage effect. If piping or a hose is attached to this assembly and run to a point of higher elevation, the backpressure will keep the air inlet valve closed because of the pressure created by the elevation of water. Hence, it would not provide the intended protection. Therefore, this type of assembly must always be installed at least six (6) inches above all downstream piping and outlets. Additionally, this assembly may not have shut-off valves or obstructions downstream. A shut-off valve would keep the assembly under pressure and allow the air inlet valve (or float check) to seal against the air inlet port, thus causing the assembly to act as an elbow, not a backflow preventer. The AVB may not be under continuous pressure for this same reason. An AVB must not be used for more than twelve (12) out of any twenty-four (24) hour period. It may be used to protect against either a pollutant or a contaminant, but may only be used to protect against a back-siphonage condition.

Automatic drain valve Spring loaded valve that automatically opens and drains the line when the pressure drops to near zero.

Available soil moisture Difference at any given time between the actual soil moisture content in the root zone soil and the wilting point.

Available water Portion of water in a soil that can be readily absorbed by plant roots.  It is the amount of water released between in situ field capacity and the permanent wilting point. 

Average annual precipitation Long-term historic (generally 30 years or more) arithmetic mean of precipitation (rain, snow, dew) received by an area.

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